The distinct sex phenotypes of male and female hinge on the development of the fetal gonads as testes or ovaries, in turn, regulated by the molecular genetic machinery of sex determination. Here, I discuss five aspects of mammalian gonadal development that distinguish it from other examples of organogenesis, and continue to surprise and fascinate.

Let's face it: males and females are very different animals—so much so, that for any species there are really two developmental biologies, not one. Humans have been intrigued by the differences between men and women since the beginning of recorded history, and presumably long before. As a developmental biologist, it is especially fascinating to ask how the differences between the sexes arise. Finding the answers involves a stimulating mix of molecular genetics, cell biology, and developmental anatomy. Since our sex phenotype depends critically on the formation of testes or ovaries in the embryo, research efforts focus largely on the genetic control of sex determination and the organogenesis of the gonads. After half a lifetime, I am still busy delving into these issues. In this chapter, I attempt to rationalize this enduring fascination by describing five aspects of sex development that continue to captivate.